The heart of a computer is a magnetic disk drive which typically includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and/or write heads over selected data tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating (in some disk drives, there is a load/unload ramp so contact with the disk does not occur); but, when the disk rotates, air is swirled by the rotating disk adjacent a media facing surface of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The ongoing quest for higher storage bit densities in magnetic media used in disk drives has reduced the size (volume) of data cells to the point where the cell dimensions are limited by the grain size of the magnetic material. Although grain size can be reduced further, there is concern that data stored within the cells is no longer thermally stable, as random thermal fluctuations at ambient temperatures are sufficient to erase data. This state is described as the superparamagnetic limit, which determines the maximum theoretical storage density for a given magnetic medium. This limit may be raised by increasing the coercivity of the magnetic medium or lowering the temperature. Lowering the temperature is not a practical option when designing hard disk drives for commercial and consumer use. Raising the coercivity is a practical solution, but requires write heads employing higher magnetic moment materials which will make data recording more challenging.
One solution has been proposed, which employs heat to lower the effective coercivity of a localized region on the magnetic medium surface and writes data within this heated region. The data state becomes “fixed” upon cooling the medium to ambient temperatures. This technique is broadly referred to interchangeably as “heat assisted magnetic recording”, HAMR, or “thermally assisted (magnetic) recording”, TAR or TAMR. HAMR can be applied to both longitudinal and perpendicular recording systems, although the highest density state of the art storage systems are more likely to be perpendicular recording systems. Heating of the media surface has been accomplished by a number of techniques such as focused laser beams or near field optical sources. U.S. Pat. No. 6,999,384 to Stancil et al., which is herein incorporated by reference, discloses near field heating of a magnetic medium.
The heat used in HAMR is provided by a plasmonic nanostructure, namely an NFT, which locally elevates a limited spot on the medium to its Curie temperature of about 600° C. Thus, the thermal and mechanical reliability of the NFT is important. However, the thermo-mechanical response of the NFT when exposed to such high temperatures leads to an undesirable protrusion of the plasmonic structure, thereby making the NFT the minimum fly point over the medium. This increases the risk of damage to the NFT resulting from disk contact during touchdown and the presence of high thermal asperities during back-off operations. However, previous attempts to protect NFTs from such damage have been unable to do so with a desired amount of efficiency.